D,L-methionine is a sulfur-containing amino acid essential to human body and is closely related to metabolism of various sulfur-containing compounds in biological organisms. Methionine belongs to the first restrictive amino acid and cannot be synthesized inside animal body and needs to be taken from the food. Adding methionine into the feedstuffs can promote growth of livestocks, increase lean meat percentage, and shorten the feeding cycle. Methionine can also be used for prevention and treatment of liver diseases such as chronic or acute hepatitis and hepatic cirrhosis because methyl group contained in methionine can be used for methylation of toxicants or medicines and thus have detoxification effect. Methionine can also be used for alleviation of toxic reaction of hazardous substances such as arsenic, trichloride, carbon tetrachloride, benzene, pyridine and quinoline. According to relevant statistical information, market demand for methionine worldwide has reached 1,000,000 ton/year. The demand for methionine in international market is increasing at a year-to-year growth rate of 4% in recent years; whereas the demand in China is increasing at a year-to-year growth rate of 7%.
As introduced in relevant literatures, there are following methods for synthesis of D,L-methionine:
In the patent publication with the publication number of CN1923807A, Sumitomo Chemical Co., Ltd. from Japan proposes to prepare D,L-methionine through hydrolysis of 5-(β-methylthioethyl)glycolyurea with Alkaline potassium compounds, then acidification with pressurized CO2 gas, and then fractionized condensation and crystallization with addition of polyvinyl alcohol. Although such method can recycle CO2 gas, due to factitious addition of polyvinyl alcohol, the crystallization mother solution as discharged cannot be recycled and can only be wasted, which produces a large quantity of wastewater S and N containing organic compounds. Furthermore, fractionized condensation method consumes a large quantity of thermal energy to the extent of resulting in significant increase in product cost for D,L-methionine.
In the patent publication with the publication number of CN85108505A, Wang Jianhua proposes to add a mixture of neutral amino acid and organic acid, containing 10% acrolein, as the catalyst during preparation of 5-(β-methylthioethyl)glycolyurea; it aims to prepare 5-(β-methylthioethyl)glycolyurea with a one-step method by using methanethiol and acrolein, simplify operation process, and improve preparation yield of 5-(β-methylthioethyl)glycolyurea. However, a mixture of neutral amino acid and organic acid that is intentionally added for further saponification with the addition of alkaline substances inevitably participates in the reaction, and produces metal salt. As a result, the metal salts accumulated in the mother solution so that the mother solution cannot be recycled and produces a large quantity of wastewater S and N containing organic compounds.
In the patent publication with the publication number of CN85108531A, Wang Jianhua proposes to produce D,L-methionine through saponification of 5-(β-methylthioethyl)glycolyurea with alkaline sodium compounds, acidification with vitriol and separation with fractionized condensation and crystallization method. Such method may produce a large quantity of by-products such as sodium sulfate, separation and processing of sodium sulfate will become a significant burden for production.
In the patent publication with the publication number of CN1589259A, to obtain a methionine preparation method that can stably produce a granular or tabular methionine crystal of high bulk density and quality, Nippon Soda Co., Ltd. proposes to use one intermetallic compound selected from metal hydroxide, metal carbonate or metal carbonate salt to hydrolyze 5-(β-methylthioethyl)glycolyurea to obtain the methionine metal salt, then neutralize the methionine metal salt by using pressurized CO2 gas to realize crystallization of methionine. After that, the methionine is separated from the filtrate, and one makes sure that the filtrate can be recycled for hydrolysis of 5-(β-methylthioethyl)glycolyurea during preparation of methionine. According to this method, despite of the fact that partial mother solution for crystallization is again used for hydrolysis, some tailings and by-products are not eliminated during intermediate process and are inevitably accumulated to the extent of affecting application effect and product quality. Meanwhile, because the mother solution is not used for preparation of 5-(β-methylthioethyl)glycolyurea, a large quantity of water is required, causing the water amount cannot be balanced over the whole process. Furthermore, each use will consume a large quantity of water as required by preparation of 5-(β-methylthioethyl)glycolyurea.
In the patent publication with the publication number of CN1103066A, Degussa AG proposes a synthesis technique to obtain an amino nitrile compound through reaction among methylthio propionaldehyde, HCN and ammonia, and hydrolyze amino nitrile compound with a ketone catalyst to obtain the aminoamide, and then finally prepare methionine through high-temperature hydrolysis with an alkaline catalyst. Such methionine synthesis method also has some defects despite of the fact that it is provided with unique features as compared with industrialized production method, The most serious defect lies in a large quantity of wastewater as produced; for instance, aminoamide obtained through addition of ketone catalyst during its preparation shall subject to separation and purification by passing through column, such separation and purification method is inappropriate for such bulk product during industrialized production. Viewed from overall techniques, it fails to solve the problem of a large quantity of waste water.
In the patent publication with the publication number of CN102399177A, Li Kuanyi proposes a green and clean technical method for continuous synthesis of methionine: taking methylthio propionaldehyde as synthesized with acrolein and methanthiol as the material for reaction with hydrogen cyanide to obtain the intermediate 2-hydroxy-4-(methylthio)nitrile; further obtaining hydantoin solution through continuous reaction of the intermediate 2-hydroxy-4-(methylthio)nitrile in the first reaction bed of combined reactor in the presence of excessive ammonia and carbon dioxide. The hydantoin solution will flow out of the first reaction bed, and release excessive carbon dioxide and ammonia in the desorption column. The hydantoin solution subjecting to desorption will flow in the second reaction bed of the combined reactor for hydrolysis under alkaline conditions to obtain the methionine potassium solution. The methionine potassium solution with carbon dioxide are neutralized to obtain methionine and potassium hydrogen carbonate solution. Methionine is separated from the solution by means of crystallization; whereas potassium hydrogen carbonate and mother solution shall be subject to further treatment for recycling. It appears that this technical approach is extremely clean and appropriate for industrialized production. However, according to its patent introduction and analysis from the angle of industrialized production, it still has some disadvantages. First, such technique requires a large quantity of water for reaction during preparation of hydantoin solution; whereas mother solution for crystallization is used for hydrolysis other than preparation of hydantoin. As a result, a large quantity of water is distilled for preparation of hydantoin during hydrolysis to further apply remaining mother solution for hydrolysis of hydantoin. This process results in increased production cost because it requires a large amount of thermal and electric energies. Second, delayed elimination of by-products as produced during chemical reaction in the whole technological process may seriously affect the quality of final products, and making it more difficult for purification of final products. Meanwhile, it also affects the consumption and batch of mother solution. This results in increased emission of wastewater during industrialized production.
As proposed in US2004/0039228A1, D,L-methionine can be obtained through reaction among some mother solution for crystallization, certain amount of NH3 and CO2 at the temperature of 60° C. in the presence of TiO2, the catalyst, saponification at the temperature of 180-300° C. and neutralization with CO2. Despite of the fact that such method makes use of partial crystallization mother solution indiscriminately to minimize the emission of wastewater, it will result in the following problems due to addition of as TiO2 catalyst in the technological process: if all mother solution for crystallization is used indiscriminately, it will result in accumulation of TiO2; Furthermore, D,L-methionine subjecting to crystallization and filtration will also contain TiO2 and thus the product will contain metal Ti that will result in reduced product quality, which makes follow-up treatment more complicated and difficult.
In the patent publication with the publication number of EB1761074A1(CN101602700A), Sumitomo Chemical puts forward the following method: using alkaline potassium compound as the catalyst for saponification of 5-(β-methylthioethyl)glycolyurea; using CO2 for neutralization and crystallization; using crystallization mother solution subjecting to aforesaid fractionized concentration and crystallization for saponification of reaction fluid of follow-up 5-(β-methylthioethyl)glycolyurea indiscriminately; adding polyvinyl alcohol into the remaining mother liquor subjecting to secondary concentration for crystallization. At this point, mother solution is abandoned other than indiscriminate use. Such technique is used by Sumitomo Chemical for industrialized production of D,L-methionine. However, there are numerous problems with this technique, especially on the aspect of green and clean production. Only partial crystallization mother solution is used. Most of crystallization mother solution is abandoned after one-time use, which produces a large quantity of wastewater.